Centrifugal compressor

ABSTRACT

A centrifugal compressor includes an impeller configured to discharge a fluid that flows into one side in an axial direction from an outlet that is directed outward in a radial direction, and a casing installed to surround the impeller and having an outlet flow channel configured to increase a pressure of the fluid as the fluid flows downstream while the fluid discharged from the outlet of the impeller flows therethrough, wherein a circulation flow channel having a first opening section connected to the outlet flow channel and a second opening section connected to an upstream side of the first opening section in the outlet flow channel.

TECHNICAL FIELD

The present invention relates to a centrifugal compressor configured to compress a gas using a centrifugal force.

Priority is claimed on Japanese Patent Application No. 2013-020704, filed Feb. 5, 2013, the content of which is incorporated herein by reference.

BACKGROUND ART

As is well known, a centrifugal compressor is configured to allow a gas to pass through a rotating impeller in a radial direction and compress the gas using a centrifugal force generated thereupon. In such a centrifugal compressor, a multi-stage type centrifugal compressor including impellers in an axial direction in a plurality of stages and configured to compress a gas in a stepped manner is known.

In the centrifugal compressor, the impeller is rotatably supported on a rotary shaft in a casing, and as the impeller is rotated, a fluid such as an air, a gas, or the like, is suctioned from a suction port of the casing and a centrifugal force is applied thereto. Then, kinetic energy is converted into pressure energy by a diffuser and a scroll section to be delivered from a discharge port of the casing.

In the above-mentioned centrifugal compressor, in order to limit the occurrences of suppress a rotating stall generated by uniformizing a flow in a circumferential direction, a diffuser width is set to be smaller than in the related art, or a circulation flow channel that connects an impeller back surface and a diffuser inlet is installed (for example, see Patent Literature 1 and Patent Literature 2).

Even in both of the configurations, a structure in which the rotating stall cannot be easily generated is provided by increasing a flow velocity in a diffuser inlet and further reducing the flow angle of the flow in a radial direction.

CITATION LIST Patent Literature [Patent Literature 1] Patent Literature 1: Japanese Unexamined Patent Application, First Publication No. 2011-122516 Patent Literature 2: Japanese Unexamined Patent Application, First Publication No. 2010-043648 SUMMARY OF INVENTION Technical Problem

Here, when the centrifugal compressor has the structure in which the circulation flow channel connects the impeller back surface and the diffuser inlet as disclosed in the above-mentioned Patent Literature 1 and Patent Literature 2 is installed, some of a high pressure compressed fluid is ejected to the impeller back surface and not to the diffuser, in the structure. Accordingly, a leakage flow from a main stream to the impeller back surface is reduced, and a main stream velocity is maintained. However, there is a margin for improvement in increasing the flow velocity of the fluid that flows through the outlet flow channel.

In addition, a leakage from a seal (for example, a labyrinth seal) installed between a rotary shaft and a hub casing may be increased.

An object of the present invention is to provide a centrifugal compressor capable of more securely limiting the occurrence of rotating stall by increasing the flow velocity of a fluid that flows through an outlet flow channel.

Solution to Problem

According to a first aspect of the present invention, a centrifugal compressor includes: an impeller configured to discharge a fluid introduced toward one side in an axial direction from an outlet directed to the outside in a radial direction through rotation thereof; and a casing installed to surround the impeller and having an outlet flow channel through which a fluid sent from the outlet of the impeller flows downstream thereby increasing the pressure of the fluid therein further downstream in the flow direction, wherein a circulation flow channel having a first opening section connected to the outlet flow channel and a second opening section connected to an upstream side of the first opening section in the outlet flow channel is formed.

According to the above-mentioned configuration, as the fluid introduced from the second opening section of the outlet flow channel is circulated to increase a flow velocity of the fluid flowing through the outlet flow channel, occurrences of rotating stall can be more securely limited.

In the centrifugal compressor, the second opening section may be oriented toward the downstream side of the outlet flow channel.

According to the above-mentioned configuration, since a circulation flow introduced from the second opening section is directed toward the downstream side of the outlet flow channel, a flow velocity of the fluid flowing through the outlet flow channel can be further increased.

In the centrifugal compressor, the outlet flow channel may include: a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy, and a scroll connected to an outlet of the diffuser and configured to eject the fluid to the outside, and the first opening section may be formed at the scroll.

According to the above-mentioned configuration, since a pressure difference between the first opening section and the second opening section can be further increased, the circulation flow can more securely flow.

In the centrifugal compressor, the outlet flow channel may include a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy, and the first opening section may be formed at an outlet side of the diffuser.

Advantageous Effects of Invention

According to the present invention, as the fluid introduced from the second opening section of the outlet flow channel is circulated to increase the flow velocity of the fluid that flows through the outlet flow channel, occurrences of rotating stall can be more securely limited.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a centrifugal compressor of an embodiment of the present invention.

FIG. 2 is an enlarged view showing an impeller and a scroll section of a final stage in the centrifugal compressor of the embodiment of the present invention.

FIG. 3 is an enlarged view showing an impeller and a scroll section of a final stage in a centrifugal compressor in a modification of the embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention is described in detail with reference to the accompanying drawings. Further, in the embodiment, as an example of a centrifugal compressor, a multi-stage type centrifugal compressor having a plurality of impellers will be exemplarily described.

As shown in FIG. 1, a centrifugal compressor 1 of the embodiment is generally constituted by a rotary shaft 2 rotatable about an axis O, an impeller 3 attached to the rotary shaft 2 and configured to compress a fluid G such as an air or the like using a centrifugal force, and a casing 5 configured to rotatably support the rotary shaft 2 and having a flow channel 4 through which the fluid G flows from an upstream side toward a downstream side.

The casing 5 is configured to form a substantially columnar contour, and the rotary shaft is disposed to pass through a center of the casing 5. In the casing 5, journal bearings 7 are installed at both ends of the rotary shaft 2 in the axial direction, and a thrust bearing 8 is installed at one end thereof. The journal bearings 7 and the thrust bearing 8 rotatably support the rotary shaft 2. That is, the rotary shaft 2 is supported by the casing 5 via the journal bearings 7 and the thrust bearing 8.

In addition, a suction port 9 configured to introduce the fluid G from the outside is formed at one end side of the casing 5 in the axial direction and a discharge port 10 configured to discharge the fluid G to the outside is formed at the other end side. In the casing 5, an internal space 11 in communication with the suction port 9 and the discharge port 10 and having a diameter which is repeatedly increased and a decreased is provided. The internal space 11 functions as a space for accommodating the impeller 3 and also functions as the above-mentioned flow channel 4. That is, the suction port 9 and the discharge port 10 are in communication with each other via the impeller 3 and the flow channel 4. In addition, the casing 5 is constituted by a shroud casing 5 a and a hub casing 5 b, and the internal space 11 is defined by the shroud casing 5 a and the hub casing 5 b.

The plurality of impellers 3 are arranged in the axial direction of the rotary shaft 2 at intervals. Further, in the example shown, while six impellers 3 are installed, at least one impeller may be installed. As shown in FIG. 2, each of the impellers 3 is constituted by a substantially disk-shaped hub 13 having a diameter gradually increased toward the discharge port 10, a plurality of blades 14 radially attached to the hub 13 and disposed in a circumferential direction in parallel, and a shroud 15 attached to cover front end sides of the plurality of blades 14 in the circumferential direction.

The flow channel 4 is formed to advance in the axial direction and connect the impellers 3 while meandering in the radial direction of the rotary shaft 2 such that the fluid G is compressed by the plurality of impellers 3 in the stepped manner Specifically describing, the flow channel 4 is generally constituted by a suction passage 17, a compression passage 18, a diffuser passage 19, and a return passage 20. The diffuser passage 19 is a passage configured to convert kinetic energy applied to the fluid by the impellers 3 into pressure energy.

The suction passage 17 is a passage configured to convert the direction of the fluid G into the axial direction of the rotary shaft 2 immediately before the impeller 3 after the fluid G flows from the outside in the radial direction toward the inside in the radial direction. Specifically, the suction passage 17 is constituted by a straight passage 21 having a linear shape through which the fluid G flows from the outside in the radial direction toward the inside in the radial direction, and a corner passage 22 having a curved shape and configured to convert the flow direction of the fluid G flowing from the straight passage 21 from the inside in the radial direction into the axial direction to direct the fluid G toward the impeller 3.

Further, a plurality of return vanes 23 radially disposed about the axis O and configured to divide the straight passage 21 in the circumferential direction of the rotary shaft 2 are installed at the straight passage 21 disposed between the two impellers 3.

The compression passage 18 is a passage configured to compress the fluid G conveyed from the suction passage 17, in the impeller 3, and is defined by being surrounded by a blade attachment surface of the hub 13 and an inner wall surface of the shroud 15.

The diffuser passage 19 has an inside in the radial direction in communication with the compression passage 18, and functions to allow the fluid G compressed by the impeller 3 to flow outward in the radial direction. Further, while the outside in the radial direction of the diffuser passage 19 is in communication with the return passage 20, the diffuser passage 19 connected to the outside in the radial direction of the impeller 3 disposed at the most downstream side of the flow channel 4 (in FIG. 1, the sixth impeller 3) is in communication with an ejection scroll 12, which will be described below.

The return passage 20 is formed with a substantially U-shaped cross section, an upstream end side of the return passage 20 is in communication with the diffuser passage 19, and a downstream end side is in communication with the straight passage 21 of the suction passage 17. The return passage 20 reverses the flow direction of the fluid G flowing to the outside in the radial direction through the diffuser passage 19 toward the inside in the radial direction due to the impeller 3 (the impeller 3 of the upstream side), and delivers the fluid G to the straight passage 21.

The ejection scroll 12 configured to eject the fluid from the ejection port is formed in the casing 5. The ejection scroll 12 has a scroll flow channel 25 formed to surround the entire circumference of the outlet of the diffuser passage 19 disposed at an outer circumferential section of the impeller 3 of the final stage.

The scroll flow channel 25 is formed to surround the entire circumference of the outlet of the diffuser passage 19 disposed at the outer circumferential section of the impeller 3 of the final stage, and is formed to gradually and continuously enlarge a cross-sectional area thereof in a rotational direction of the impeller 3.

The diffuser passage 19 and the ejection scroll 12 function as an outlet flow channel 6 configured to increase a pressure of the fluid as the fluid is directed toward the downstream side while the fluid provided from the outlet of the impeller 3 flows therethrough.

Then, in the casing 5 of the centrifugal compressor 1 of the embodiment, a circulation flow channel 26 connecting the bottom surface of the scroll flow channel 25 of the ejection scroll 12 and the diffuser passage 19 is formed. Specifically, the circulation flow channel 26 is the flow channel 4 having a second opening section 28 formed in the vicinity of an inlet of the diffuser passage 19, and a first opening section 27 formed at the bottom surface of the scroll flow channel 25.

The second opening section 28 is formed to be directed toward the downstream side of the diffuser passage 19. That is, the second opening section 28 oriented such that the compressed air introduced from the first opening section 27 and ejected from the second opening section 28 is ejected toward the outlet of the diffuser passage 19.

Next, compression of the fluid G by the centrifugal compressor 1 configured as described above will be described.

When the impellers 3 are rotated with the rotary shaft 2, the fluid G introduced into the flow channel 4 from the suction port 9 flows sequentially into the suction passage 17, the compression passage 18, the diffuser passage 19 and the return passage 20 of the impeller 3 of the first stage from the suction port 9, and then, flows sequentially into the suction passage 17, the compression passage 18 . . . of the impeller 3 of the second stage. Then, the fluid G flowing to the ejection scroll 12 immediately after the diffuser passage 19 disposed at the most downstream side of the flow channel 4 flows to the outside from the discharge port 10.

The fluid G is compressed by the impellers 3 in the middle of flowing through the flow channel 4 in the above-mentioned sequence. That is, in the centrifugal compressor 1, the fluid G is compressed by the plurality of impellers 3 in the stepped manner, and thus, a large compression ratio can be easily obtained.

Here, the circulation flow channel 26 is configured to capture some of the compressed fluid from the first opening section 27 and eject the compressed fluid from the second opening section 28 such that some of the compressed fluid is circulated between the diffuser passage 19 and the ejection scroll 12.

According to the embodiment, the compressed air is introduced from the first opening section 27 of the scroll flow channel 25 to be ejected from the second opening section 28 of the diffuser passage 19 via the circulation flow channel 26. That is, the compressed air is circulated to increase a flow velocity of the fluid flowing through the diffuser passage 19 with the compressed fluid. Accordingly, occurrences of rotating stall can be more securely limited.

In addition, as the second opening section 28 is formed to be oriented toward the downstream side of the diffuser passage 19, since the compressed fluid introduced from the second opening section 28 is ejected toward the downstream side of the diffuser passage 19, the flow velocity of the fluid flowing through the diffuser passage 19 can be further increased.

In addition, as the first opening section 27 is formed at the ejection scroll 12, since a pressure difference between the first opening section 27 and the second opening section 28 is further increased, a circulation flow can more securely flow.

Further, since no compressed air is introduced onto the back surface of the impeller 3, for example, when the seal such as a labyrinth seal or the like is installed between the rotary shaft 2 and the casing 5, a leakage from the seal is not increased.

Further, in the embodiment, while a configuration in which the first opening section 27 of the circulation flow channel 26 is formed at the ejection scroll 12 is provided, a formed place of the first opening section 27 may be a downstream side of the second opening section 28 on the outlet flow channel constituted by the diffuser passage 19 and the ejection scroll 12. For example, as shown in FIG. 3, a configuration in which the first opening section 27 is formed at the outlet side of the diffuser passage 19 may be provided.

Further, the technical spirit of the present invention is not limited to the embodiment and various modifications may be made without departing from the spirit of the present invention.

For example, the circulation flow channel 26 may be branched off from a conventional line to eject the fluid to the diffuser passage 19 without necessity of newly forming the entire circulation flow channel.

In addition, in the embodiment, while the second opening section 28 is configured to be oriented toward a slipstream side of the diffuser passage 19, it is not limited thereto as long the flow velocity of the compressed fluid flowing through the diffuser passage 19 should not decreased. For example, the fluid may be injected in a direction perpendicular to the extending direction of the diffuser passage 19.

INDUSTRIAL APPLICABILITY

According to the centrifugal compressor, occurrences of rotating stall can be more securely limited by circulating the fluid such that the fluid introduced from the second opening section of the outlet flow channel increases the flow velocity of the fluid flowing through the outlet flow channel.

REFERENCE SIGNS LIST

-   1 centrifugal compressor -   2 rotary shaft -   3 impeller -   4 flow channel -   5 casing -   6 outlet flow channel -   9 suction port -   10 discharge port -   11 internal space -   12 ejection scroll -   13 hub -   14 blade -   15 shroud -   17 suction passage -   18 compression passage -   19 diffuser passage -   20 return passage -   21 straight passage -   22 corner passage -   23 return vane -   25 scroll flow channel -   26 circulation flow channel -   27 first opening section -   28 second opening section 

1. A centrifugal compressor comprising: an impeller configured to discharge a fluid introduced toward one side in an axial direction from an outlet of the impeller directed to the outside in a radial direction through rotation thereof; and a casing installed to surround the impeller and having an outlet flow channel through which a fluid sent from the outlet of the impeller flows downstream thereby increasing a pressure of the fluid therein further downstream in the flow direction, wherein a circulation flow channel having a first opening section connected to the outlet flow channel and a second opening section connected to an upstream side of the first opening section in the outlet flow channel is formed.
 2. The centrifugal compressor according to claim 1, wherein the second opening section is oriented toward the downstream side of the outlet flow channel.
 3. The centrifugal compressor according to claim 1, wherein the outlet flow channel includes: a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy; a scroll connected to an outlet of the diffuser and configured to eject the fluid to the outside; and the first opening section is formed at the scroll.
 4. The centrifugal compressor according to claim 1, wherein the outlet flow channel includes a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy, and the first opening section is formed at an outlet side of the diffuser.
 5. The centrifugal compressor according to claim 2, wherein the outlet flow channel includes: a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy; a scroll connected to an outlet of the diffuser and configured to eject the fluid to the outside; and the first opening section is fainted at the scroll.
 6. The centrifugal compressor according to claim 2, wherein the outlet flow channel includes a diffuser connected to the outlet of the impeller and configured to convert kinetic energy applied to the fluid by the impeller into pressure energy, and the first opening section is formed at an outlet side of the diffuser. 